Here are some of the list of well-known MAC addresses used by the Catalyst Switches:
01-80-c2-00-00-xx -- Famous BPDU'S
01-00-00-0c-00-00 -- ISL
01-00-0c-cc-cc-cc --- CDP, DISL, DTP (Dynamic Trunk proto)- all of them use the same muliticast address, the difference is in the protocol id field.
CDP == PROTOCOL TYPE 0X2000
DTP == PROTOCOL TYPE 0X2004
The reason that all of them use the same multicast address is that the only kind of packets that are allowed to go thru on a blocked port (Spanning Tree) are CDP and BPDU's.